$$\lambda $$-Semidirect products of inverse monoids are weakly Schreier extensions

Abstract

AbstractA split extension of monoids with kernel $$k :N \rightarrow G$$ k : N → G , cokernel $$e :G \rightarrow H$$ e : G → H and splitting $$s :H \rightarrow G$$ s : H → G is weakly Schreier if each element $$g \in G$$ g ∈ G can be written $$g = k(n)se(g)$$ g = k ( n ) s e ( g ) for some $$n \in N$$ n ∈ N . The characterization of weakly Schreier extensions allows them to be viewed as something akin to a weak semidirect product. The motivating examples of such extensions are the Artin glueings of topological spaces and, of course, the Schreier extensions of monoids which they generalise. In this paper we show that the $$\lambda $$ λ -semidirect products of inverse monoids are also examples of weakly Schreier extensions. The characterization of weakly Schreier extensions sheds some light on the structure of $$\lambda $$ λ -semidirect products. The set of weakly Schreier extensions between two monoids comes equipped with a natural poset structure, which induces an order on the set of $$\lambda $$ λ -semidirect products between two inverse monoids. We show that Artin glueings are in fact $$\lambda $$ λ -semidirect products and inspired by this identify a class of Artin-like $$\lambda $$ λ -semidirect products. We show that joins exist for this special class of $$\lambda $$ λ -semidirect product in the aforementioned order.